Performance evaluation of Ammonia-diesel dual direct-injection VCR engine at high load and high AER: Coupling mechanism of compression ratio and diesel nozzle diameter

This study investigates the coupling effects of compression ratio (CR) and diesel nozzle diameter (D d ) on combustion and emission processes on an ammonia-diesel dual direct-injection two-stroke engine at full-load and a 95% ammonia energy ratio (AER). The results show that increasing CR accelerates diesel combustion and early combustion of ammonia; however, it suppresses mid-to-late combustion of ammonia. Moreover, this negative effect is further exacerbated by larger D d . The maximum CR are 15, 17 and at least 18 for D d of 0.3, 0.15 and 0.09 mm, respectively. At CR ≤ 15, higher CR and larger D d improve indicated thermal efficiency (ITE) and reduce INH 3 emissions, yet ITE remains relatively low accompanied by elevated peak firing pressure (PFP) and INOₓ emissions. At CR ≥ 18, PFP, peak pressure rise rate (PPRR) and INH 3 emissions become excessively high. At high CRs, a moderate reduction in D d helps balance diesel ignition characteristics and its continuous ignition effect on ammonia, thus improving ITE and reducing INH 3 emissions, whereas an excessively small D d triggers combustion mode transition and a sharp increase in PPRR. Two optimal strategies are recommended: CR15 with a 0.3 mm D d enables high ITE and well-controlled PFP, PPRR and emissions, while CR17 with a 0.15 mm D d achieves improved ITE and most emission performances but ammonia/diesel injection strategies need to be further optimized. This research reveals the coupling mechanism of CR and D d at full load and high AER, providing a theoretical basis for the realization of high-efficiency and low-carbon combustion in engines.